(1) Field of the Invention
The present invention relates to a system for exhaust gas purification. More particularly, the present invention relates to a system for exhaust gas purification, which can effectively purify harmful substances in exhaust gas, particularly the hydrocarbons, etc. generated in large amounts during cold engine start up.
(2) Description of the Prior Art
In the FTP test (LA-4 mode) conducted in connection with the intensified U.S. regulation for automobile exhaust gas emission, 70 to 80% of the total amount of hydrocarbons (HC) emitted during the test is allowed to be emitted within 140 seconds (within the first hill of Bag 1) after engine cranking. During this period, however, when conventional three-way catalysts are used, most of the hydrocarbons are discharged without being purified because the catalysts need a considerably long time until they reach their activation temperatures.
In order to solve this problem, there were proposed various systems for exhaust gas purification, which comprises a catalyst and an adsorbent composed mainly of zeolite, both provided in the exhaust pipe of automobile and wherein the function of the adsorbent is to adsorb hydrocarbons until the catalyst reaches its activation temperature. For example, Japanese Patent Application Kokai (Laid-Open) No. 75327/1990 proposed a system for purification of automobile exhaust gas, using a Y-zeolite or mordenite as a hydrocarbons adsorbent; Japanese Patent Application Kokai (Laid-Open) No. 293519/1992 proposed a system using an adsorbent obtained by subjecting a ZSM-5 to ion exchange with Cu and Pd; and Japanese Patent Application Kokai (Laid-Open) No. 31359/1993 proposed an adsorbent containing a high-silica zeolite having a Si/Al ratio of 40 or more and, as necessary, a heat-resistant oxide and a noble metal(s) such as Pt, Pd and/or Rh.
The zeolites used as an adsorbent in conventional systems for exhaust gas purification, however, have no sufficient heat resistance or adsorption capability and the effects are not satisfactory. Particularly with in-line type exhaust gas purification systems, since each component of system is exposed to an exhaust gas of high temperatures, there takes place, for example, the thermal destruction of zeolite crystal structure, which has tended to invite deterioration of adsorption capability.